Few senators wait until their 80s, or the start of their third decade in office, to have their breakout moment. But that’s what this past year has been for Dianne Feinstein.

At the end of last winter, the California Democrat surged to national renown as the most passionately vocal and dogged lawmaker in the uphill pursuit of the strictest new gun controls in more than a generation. The attention, both laudatory and condemning, was more than what most members receive in any one Congress. But now Feinstein is on course to outdo herself, with her blockbuster accusation that the CIA spied on Congress and intimidated her staff in an effort to hobble an oversight investigation into the agency’s former detention and interrogation program.

The twin crusades, which now stand to define the pinnacle of her prominence, are closely allied in one important way: Both have Feinstein playing against type, deploying blistering rhetoric and challenging hidebound practices in sharp contrast to her reputation, which is for level-headedness and deliberation.

At the same time, the two causes are polar opposites: Gun control has been a priority for the senator since 1978, when she ascended to the mayoralty of San Francisco after the incumbent, George Moscone, was assassinated. But becoming an outspoken critic of the clandestine community is an entirely new role for Feinstein; as chairwoman of the Senate Intelligence Committee for more than five years, she has positioned herself as one of the CIA’s most loyal defenders at the Capitol.

It’s that forceful reversal that may prove more lastingly important. Feinstein’s bid to ban many styles of semiautomatic weapons , or at least intensify background checks on would-be gun buyers, has been totally stymied at least until after the midterm elections — and that will be true even if another mass killing raises national concern about firearms violence.

But whether to tighten the reins on the sprawling intelligence-gathering community is still an open question in this Congress. It’s one of the few debates that has GOP conservatives and Democratic liberals finding common cause against the establishment mainstream, which has generally succeeded in the post-9/11 era at giving the spymasters broad latitude and billions of dollars to combat terrorism as assertively and secretly as they see fit.

Feinstein has remained at the forefront of that effort as the volume of congressional criticism has increased in recent years. She has publicly defended the CIA’s use of armed drone strikes to kill suspected terrorists in Pakistan and Yemen, supported the FBI’s assertive use of its new investigatory powers under the Patriot Act and praised the expansive telephone and Internet surveillance programs at the National Security Agency, which she credits with stopping terrorist attacks in the United States.

The CIA, in other words, has perhaps picked the worst possible member of Congress to antagonize. Her conversion from ally to combatant could hardly come at a worse time for the agency. That’s because she has the power to cause the biggest rupture between the Hill and the spies since the 1970s, when exposure of their cloak-and-dagger excesses prompted the birth of the current congressional oversight system.

The American Civil Liberties Union, which has criticized Feinstein plenty in the past for being too cozy with the agencies she oversees, hailed her for a “forceful, necessary and historic defense of the constitutional principle of separation of powers” in her Tuesday speech .

She accused the CIA of criminal activity in searching a computer network set up so that her committee aides could review top secret files as part of a Senate inquiry. (CIA Director John O. Brennan has denied her allegations and wants the Justice Department to file its own criminal charges, alleging committee staffers improperly obtained top secret files.)

The charges and counter-charges are the first public eruption in a tussle between Feinstein and the CIA that has been fomenting since 2009, when early in her chairmanship she launched an investigation of the agency’s secret prisons and harsh interrogation techniques.

Allies say the senator works to maintain a slow internal burn rate in order to magnify the impact in the rare cases where she allows her passions to bubble over. Critics suspect she suffers from a passivity that can border on indecisive paralysis.

If that’s an unfair conclusion with respect to her work as legislator and overseer, it’s perhaps more justified when it comes to her political ambitions. Her betrayal of ambivalence about running for vice president in 1984 helped prompt Walter F. Mondale to switch his choice late in the process to Rep. Geraldine Ferraro. She publicly agonized about running in a special House election in 1987 — her “no” decision at the last minute easing Nancy Pelosi’s first bid for public office. And while serving as a senator since 1993, she has had extended open flirtations with running for governor no fewer than three times, most recently four years ago.

Feinstein will become the fourth most senior Democrat in the Senate in 2015. (Still three months shy of her 81st birthday, she has also become the youngest “oldest senator” in at least the past four decades, since the deaths of a pair of her colleagues who were nearing 90, Democrats Frank R. Lautenberg of New Jersey and Daniel K. Inouye of Hawaii.)

With her career moving inevitably toward its final chapters — and with who-spied-on-whom stories remaining in the news — intelligence agency officials would do well to note the simple motto that Feinstein claimed a year ago, when it became clear her gun control legislation would get no further than committee. “I don’t give up,” she said.